Thermoplastic rubbers are relatively inexpensive and widely employed in industries where abrasion resistance is highly needed. Black styrene butadiene rubber (SBR) sheets are used as conveyor belt, belt wipers also for sealing strips, general construction mounting pads, in gasket applications and also in a large number of applications such as (e.g. footwear, adhesives manufacturing, molded or extruded goods). However, due to the non-polar nature of this rubber and indeed elastomers as a class of materials, it is difficult to achieve bonding between elastomers. Vulcanized rubbers are especially difficult to bond due to low molecular weight ingredients therein that tend to migrate to the rubber surface limiting interaction with the adhesive.
Polyurethane (PU) adhesives and sealants have for years played a significant role in numerous industrial applications due to their advantageous properties such as high hardness, modulus and excellent mechanical properties. However, passage of the Clean Air Act Amendments in 1990 in the United States and similar legislation to restricted exposure to many chemicals including components associated with PU. However, the bonding of elastomeric substrates remains challenging with PU adhesives owing to the difficulty in bonding and the elongation requirements on any such bond. As a result, it is conventional to the art to use a surface treatment to effectively bond adhesives to elastomers. The treatment is typically accomplished by sulfuric acid etching or by the application of a primer such as those described in U.S. Pat. No. 4,485,135 or U.S. Pat. No. 4,485,136. A well accepted primer for use on SBR (styrene-butadiene rubber) and the like is a 2% solution of 1,3-dichloro-5,5-dimethyl hydantoin in methyl chloride.
Surface treatments have proven to be suitable for the improvement of adhesion and wettability properties of non-polar synthetic rubbers. The adhesion of rubber has improved by changing the rubber composition or by modifying surfaces of the rubber surface by the use of a chemical agent (halogenation, cyclization, etc.) or using high energy irradiation such as bombarding the surface by electron beam or gamma irradiation to otherwise increase the number of surface bonding sites per unit area on the rubber surface. While these costly surface modifications are readily integrated into the production of new products, field repairs relying on such solutions or radiation are difficult to perform reliably and extend the duration of repairs. Additionally, many solvents are toxic to handle and may be prohibited from certain areas such as food or pharmaceutical production facilities. Another limitation of surface activation is the limited duration of such activity before the surface energy reverts to pre-treatment levels.
Commercially available adhesives for bonding rubber surfaces have met with limited acceptance owing to the need for a primer or a pre-wetting step in order to achieve adhesion, inclusion of organic solvents, low strength bonding compared to rubber to rubber vulcanization and as a result rapid adhesive failure. The reliance of existing adhesives on a primer to ensure a good adhesion poses significant problems with substrates that have distinctly uneven surfaces as the adhesive fails to fill spaces uniformly. Similarly, rigid substrates tend to have adhesive voids that weaken the strength of the adhesive bond.
Thus, there remains a need for an adhesive for bonding non-polar elastomeric surfaces that exhibits excellent adhesion without the necessity of resorting to primers or other surface treatments that change the surface energy. There further exists a need for such an adhesive to be amenable to application without resort to solvents, as solvents tend to pose health concerns and degrade bond properties.